tenthplanet

The diameter of 2003 UB313 compared with that of Pluto, its moon Charon, Earth, and the moon. (Courtesy: University of Bonn)

An icy rock discovered last year is larger than Pluto, a finding that fuels an ongoing debate about what defines a planet.

Astronomers announced they had discovered the object, informally called UB313, last summer.

It is the most distant object to be seen in the solar system.

In Thursday's issue of the journal Nature, researchers report UB313 is around 3,100 kilometres in diameter, making it larger than Pluto's 2,000 kilometres. Pluto is the smallest body traditionally called a planet in our solar system.

"It is now increasingly hard to justify calling Pluto a planet if UB313 is not also given this status," study author Frank Bertoldi of the University of Bonn said in a statement.

The discovery of UB313 led some scientists to defend Pluto's status as a planet and dismiss UB313 as a rock.

A special panel set up by the International Astronomical Union is reviewing competing definitions of the term planet.

For now, objects like UB313 are technically known as Kuiper Belt Objects or KBOs. It's estimated there are 10,000 such icy rocks moving around the sun beyond the orbit of Neptune.

"Whichever way you care to count them, with the discovery and measurement of the size of ... UB313 there are no longer nine major planets in the solar system," wrote Scott Sheppard, a terrestrial magnetism expert at Carnegie Institution of Washington in a journal commentary.

As instruments and software improve, "there may be further surprises in store" for how we understand the solar system, Sheppard said.